Electronic disorder, spin glass and large magnetoresistance in FeSr$_{2}$Y$_{2-y}$Ce$_{y}$Cu$_{2}$O$_{8+x}$

We have successfully synthesized FeSr$_{2}$Y$_{2-y}$Ce$_{y}$Cu$_{2}$O$_{8+x}$ (Fe1222) with a wide range of Ce and oxygen concentrations. Fe1222 belongs to an interesting group of compounds that contains a 2D-like CuO$_{2}$ layer and an oxygen deficient FeO$_{x}$ layer. They are structural very similar to the well-studied superconducting and magnetically ordered RuSr$_{2}$R$_{2-x}$Ce$_{x}$Cu$_{2}$O$_{10-x}$ (Ru1222). However, we do not observe superconductivity in Fe1222 and there is a spin-glass transition with antiferromagnetic exchange interactions arising from the disordered FeO$_{x}$ layer at $\sim$25 K, which does not depend on the Ce or oxygen concentration. The electronic transport in the oxygen reduced samples is highly disordered and involves variable range hopping between localized states where there is a large negative magnetoresistance of $\sim$-22\% at 8 T. The oxygen saturated samples are highly conducting at room temperature and display weak localization at low temperatures. The absence of superconductivity may be due to pair-breaking by a small Fe fraction in the CuO$_{2}$ planes.